The problem of treating patients with heart failure is gaining increasing attention. Such heart failure may be manifested as left ventricular dysfunction (LVD), congestive heart failure (CHF), or other forms. For patients with LVD, the ejection fraction from the left ventricle is reduced, and the ability of the stroke volume to respond to exercise is limited. While the disease of the heart may proceed in different ways, the result is a degenerative reduction in cardiac output. For example, LVD may be produced by a degree of left bundle branch block, leading to loss of synchrony between the left atrium and the left ventricle. Whatever the progress of the disease, cardiac output is reduced, and the heart responds so as to increase cardiac output toward a normal level primarily by heart rate increase. The rate increase may be manifested in several different ways, e.g., there may be an increase in the daily lower spontaneous rate, i.e., the nighttime rate when the patient is at rest. Further, the changed conductive patterns which may form the basis of the LVD can lead to changes in the cardiac signal, e.g., an increased width of the depolarization signal, or QRS. Further, since the body is calling for an increase in blood supply, patients with LVD are also known to have elevated levels of catecholamines at rest, which induces a higher heart rate at rest. In turn, the QT interval for such a patient is affected by the catecholamine level, and thus has a changed pattern during exercise as well. These patients have a decreased QT response, or smaller change in QT, during exercise, such that the QT interval shortening during exercise is smaller than that found normally. Although QT interval is influenced independently by heart rate alone, as well as by exercise and catecholemines, it is not known to what extent each of these factors or both are responsible for the changed QT response to exercise in LVD patients. However, it is known that patients with LVD clearly have a different pattern of QT interval shortening during exercise.
As a consequence of the above observations, the basis for this invention is obtaining information from cardiac signals for deriving an indication of LVD, or onset of LVD. It is seen that for an implanted device such as a cardiac pacemaker, which can sense cardiac signals and process heart rate (or R--R interval), QT interval, and QRS length, as well as QRS and P-wave morphology, information can be obtained for tracking changes in left ventricular function, and in particular for indicating progression toward some degree, or form of left ventricular dysfunction. In particular, a rate responsive pacemaker which uses QT interval as the rate indicating parameter, such as pacemakers made by Vitatron Medical, the assignee of this invention, can be adapted to provide information relating to left ventricular hemodynamic performance, and for indicating the onset of LVD. In such a QT rate responsive pacemaker, there is provided an algorithm for automatically adjusting the rate responsive parameters, i.e., the correlation function between QT and desired rate. This adjustment is suitably done on a daily basis where the pacemaker measures a slope of the correlation function at the lower rate limit, and adjusts the QT-rate function between lower rate limit (LRL) and upper rate limit (URL) accordingly. See U.S. Pat. No. 4,972,834, incorporated herein by reference. If such changes are stored and analyzed for a trend, progress toward LVD can be indicated. Likewise, if it is found that the patient heart rate is not dropped to the programmed LRL during nighttime, such that the spontaneous lower rate has had an upward progression, this trend can also be used as an indicator of the onset of LVD.
Since LVD is generally symptomatic of cardiac conduction disorders, other variations in sensed cardiac signals may also be used for making an indication of LVD, or for corroborating progression toward LVD. For example, in a patient with onset of LVD, the evoked T-wave amplitude may not increase in a normal manner during exercise. Thus, in normal patients with healthy hearts the evoked T-wave amplitude increases up to about fifty percent during exercise, an increase which may correlate with stroke volume. A change in the variation of T-wave amplitude with respect to exercise, and consequent heart rate, can be easily measured and tracked in a QT rate responsive pacemaker, or any pacemaker adapted to sense and recover the T-waves. Likewise, as noted above, changes in QRS duration (width) and/or morphology may also be detected and tracked for detection of a trend.
What is needed in patients with progressive LVD or other types of heart failure is the availability of data from which a cardiologist can properly and accurately chart the progression, determine the nature of the heart failure, and be able to implement the optimal treatment in a timely fashion. Thus, physicians need a better means of diagnosing development of, or onset of LVD, in order to provide appropriate treatment. Modern pacemakers, and indeed even 4 chamber pacemakers for treating heart failure by bi-ventricular pacing, have heretofore not provided any such means for collecting data concerning the onset of LVD. Rather, the procedure has been essentially limited to direct examinations of the patient, which are time consuming and costly, and which cannot be repeated with a desired frequency so as to provide accurate trend information.
Another problem existing in the art today is that patients with implanted DDD pacemakers who develop LVD, may be in a situation where ventricular pacing produces a deleterious effect. Thus, if the pacing may cause a mechanical activation sequence which is abnormal, in which case it would reduce the cardiac output. For example, in patients having left bundle branch block, it has been reported that with normal pacing there may be risk in aggravating activation sequences. See "Four Chamber Pacing in Dilated Cardiomyopathy," Cazeaq et al., PACE, Vol. 17, November 1994, Part II.
In view of the above, it is seen that there is a substantial need in the art for a pacemaker or other implanted device having the capacity to identify the onset of cardiac heart failure, and in particular LVD, and to provide such indication to the patient and the patient's physician, or to provide for an appropriate automatic response to treat the changing patient condition.